1. Field of the Invention
This invention relates to improvements in stacked multicell batteries, and more particularly to preventing ionically conductive paths from forming between adjacent cells and the battery case.
2. Description of the Prior Art
When constructing practical electrochemical cells in batteries there are two basic ways in which electrodes can be connected inside the cell or battery module case. These are series and parallel connections. In a bipolar battery design, the electrodes are hooked together in series, thus the voltage of the stack is n times that of a single cell, where n is equal to the number of cells in the stack. Each cell being comprised of a positive and a negative electrode separated from one another by some electronically insulative material.
To build a long life bipolar cell stack requires that there be no electrolyte path (i.e., no ionic leakage) formed between adjacent cells around the bipolar wall. This ensures that all of the current flow through the bipolar wall should be electronic in nature and there should be no ionic flow. Thus, the practical problem in building bipolar cell stacks is how to prevent liquid electrolyte paths from developing between adjacent cells. Because of the inability to completely contain the electrolyte within each cell, maximum performance and life of rechargeable bipolar batteries has not yet been realized to date.
Referring to FIG. 1 which shows a prior art cell stack, the positive and negative electrodes and separator of each cell contain an electrolyte which is liquid at the operating temperature. The bipolar wall shown in FIG. 1 separates adjacent cells and is designed to allow an electronic path between adjacent cells while not allowing an ionic path. If a path of electrolyte is allowed to travel around the bipolar wall, an ionic short develops reducing the effectiveness of the battery. When the liquid electrolyte contacts the bipolar wall, it may either wet or not wet the surface of the bipolar wall. When the liquid electrolyte runs across the surface of the bipolar wall, the wall is said to be wetted by the electrolyte. It is when the bipolar wall is wetted by the electrolyte that the electrolyte might travel in an undesirable path, shown in the figure, to the adjacent cell.
The ideal solution to the electrolyte leakage problem is to use a minimum amount of electrolyte which is completely contained within the individual electrodes and separator by capiliary forces. In practice, however, this is almost impossible since more than this minimum amount of electrolyte is required to obtain the desired electrochemical performance from the battery.
The critical component to make a bipolar battery feasible is the development: of a reliable bipolar wall edge seal to prevent the migration of electrolyte between adjacent cells and from the cells to the module case wall. If electrolyte bridges across adjacent cells or to the conductive battery case, ionically conductive paths will be formed which will degrade the capacity of the effected cells and result in an imbalance in the cell stack and, ultimately, failure of the entire battery.
Others have attempted to solve this electrolyte containment problem by forming a hermetic seal around the perimeter of each cell thereby permanently isolating each cell from adjacent cells and from the battery case. This approach requires a sophisticated insulating material that can withstand the high operating temperatures and yet is capable of being bonded to metal between the cell hardware components. The insulating material must be capable of withstanding chemical, electrochemical, thermal and mechanical effects imposed by the cell stack under all operating and environmental conditions. The assembly of this type of seal will result in high fabricating costs and low reliability due to the large area which must be perfectly sealed. The mechanical loading on the ceramic ring caused by thermal expansion of the components during heating and cooling may limit this type of hermetic seal design to circular configurations in relatively small sizes (i.e., approximately five inches). This may impose severe packaging penalties and reduce the energy density for many potential battery applications. Therefore, alternative means are necessary to confine the electrolyte from migrating from the electrodes and separator around the bipolar wall.